This invention relates generally to computer controlled systems, and particularly to a system for inputting a security code to such a system. Typically, computer controlled systems are provided with a security code which prevents the use of the system by anyone who does not know the code. The security code is hardwired into the computer and the operator uses a a keyboard to manually input the same code to the computer prior to using the system. Security against unauthorized use can be greatly increased by periodically changing the security code. The security code typically is changed by selecting a new code and setting the computer to respond to this code. Authorized operators are informed of the new code and input the new code to the system as the first step in using the system. For this reason, there is a need for a system which enables an operator to input any one of a large number of security codes to a computerized system. The instant invention fulfills this need by the provision of a system for individually inputting the security code pulses to a computer controlled system.